The invention is directed to an improved method and apparatus for reducing the noise associated with a diesel engine at idling speeds. In such a method, known from German Offenlegungsschrift 33 10 872, the adjustment of the cam drive varies the fuel pumping rate in a controlled manner. The method uses a radial piston injection pump, in which in cooperation with the electrically controlled valve, even the beginning of the cam lobe curve can be effectively exploited for injection. A shift is made from a low fuel injection rate, corresponding to the relatively low initial rise of the cam lobe curve, to a high fuel injection rate, corresponding to the middle range of the cam lobe curve. This is done in order to adapt both the injection onset and the injection rate to the needs of the engine, and in particular to vary them during engine operation.
Low-noise combustion of the small quantities of fuel that are injected for low-load or idling operation is a particular problem in Diesel engines. A great many proposals have been made, which to this end strive to prolong the duration of injection, which is associated with a simultaneous reduction in the injection rate. To lessen the noise, as little fuel as possible should abruptly ignite at the instant of ignition. If injection is performed in these special operating ranges at a low fuel injection rate, then only some of the fuel ignites abruptly; the rest of the as yet uninjected fuel is delivered continuously thereafter. Such provisions for prolonging the injection duration can be put into practice by revising how either the injection valve or the pump is constructed. These provisions typically entail high construction costs, especially if the reduction of the fuel injection rate is supposed to take place by diverting partial fuel quantities, bypassing the fuel quantity pumped at high pressure.